In the oil well drilling industry, it is important to reduce the economic cost of drilling a wellbore in order to extract oil and gas from underground reservoirs. With underground resources becoming accessible at even greater depths, it becomes evermore important to identify the most efficient and effective drilling configuration to be used in order to drill through the intervening rock formation and access the underground reservoir.
In order to plan any well drilling operation, it is common to conduct a preliminary study of the intervening rock formation between the surface and the underground reservoir, and to select and design a series of drill bits and drill bit configurations to be used in drilling a wellbore through the formation to the reservoir.
In any formation, there will often be a number of different types of rock, as well as one or more intervals, along the determined path of the wellbore, which provide a particular resistance to being drilled. Where such intervals can be identified, the drilling operation can be planned in advance so that drill bits capable of a high rate of penetration can be used in non-problematic sections of the wellbore, whilst specialized drill bit configurations which are more resistant to wear and have a greater cutting capacity can be used to drill through the more problematic intervals.
Nevertheless, the geological properties within any such interval will never be constant, and even in the same rock formation, the same apparent type of problematic rock interval can have markedly different constitution as between one interval and the next, both in terms of the geological composition throughout the interval, such as different proportions of different rock types within the formation, or simply a variation in the drillability of the rock, for example due to variations in the rock strength.
These natural variations in the geological properties of the formation make the prediction of drilling performance and the planning of well drilling operations difficult, and limit the accuracy with which any drilling performance can be predicted.
In order to calibrate the predictive models used to plan well drilling operations, accuracy can be improved by utilizing the results of actual drilling measurements obtained in order to compare the expected performance of a drill bit configuration against the actual performance of the drill bit configuration in use. The actual drilling results can be used to refine and improve the predictive drilling model.
Nevertheless, a drilling operator may feel more comfortable proceeding with the design and selection of drill bit configurations based on actual drilling results which have been obtained by using one or more particular drilling configurations in the field. In such situations, the drilling operator will often seek to compare the like-for-like real life performance of several different drill bit configurations, and will wish to base his selection and design of future drill bit configurations on those drill bit configurations which have proven most successful in actual drilling operations in the field.
In this situation, however, there is an inherent risk that the respective in-field performance results may be misleading as to which drill bit configuration actually provides the best performance. This problem arises due to the inherent natural variations in the geological properties of the formation, meaning that the drilling results from any two real-life drilling intervals can be difficult to compare in a simple side-by-side comparison.
Put in simple terms, if two different drill bit configurations are each used to drill a 100 m interval in a rock formation, for example in parallel wellbores, one cannot simply afterwards assess the measured rate of penetration or the actual time taken to drill through the 100 m interval in order to determine which drill bit configuration performed the best, or directly compare the extent of wear on the two bits to see which was most resistant to bit wear, as one of the two drilled intervals may have had a significantly higher proportion of a rock type which is resistant to being drilled or which produces a significantly higher degree of bit wear. Even where the constitution of the rock types in each interval is similar, one of the intervals may exhibit a significantly larger proportion of rock with high rock strength than the other interval.
It would therefore be advantageous to provide a method for assessing the performance of a drill bit for drilling an interval which takes account of the actual drilling conditions encountered, and which permits a meaningful comparison between the performances of different drill bit configurations used for drilling different intervals of the same or different wellbores.